The invention relates to a semiconductor laser having a semiconductor body comprising an active laser region having a p-n junction, in which the active region is present within a resonator which is formed by two mutually parallel reflecting non-oxidized mirror faces, in which contact members are present to apply current in the forward direction to the p-n junction to generate coherent electromagnetic radiation in the active region, and in which the active region comprises end zones adjoining the mirror faces so as to reduce non-radiating recombination near the mirror faces.
The invention relates in addition to a method of manufacturing the said semiconductor laser.
A semiconductor laser of the kind described is known from the article by Yonezu et al. in Applied Physics Letters 34 (1979) pp. 637-639.
In conventional semiconductor lasers in which the active region is present between reflecting surfaces of the crystal, also termed mirror faces, said mirror faces tend to erode in the long run under the influence of non-radiating recombination at or near the mirror faces. This tendency is particularly strong when the laser is operated in an atmosphere comprising water vapor, even when this is present only in a very small concentration (for example, 1:10.sup.5). This mirror erosion gives rise to a gradual degradation of the properties of the laser, inter alia to a continuous increase of the threshold current and to the occurrence of pulsations in the emitted radiation. See, for example, J. A. F. Peek in Electronics Letters Vol. 16 No. 11, May 22, 1980, pp. 441-442, H. Yonezu et al. in Journal of Applied Physics, 50(8), August, 1979, pp. 5150-5157, and F. R. Nash et al. in Applied Physics Letters, 35(12) Dec. 15, 1979, pp. 905-907. The cause of all this is that the reflecting properties of the mirror faces rapidly deteriorate as a result in an increase of the roughness of the said mirror faces.
Said erosion can be mitigated in various manners. For example, the water vapor can be eliminated, for example, by operating the laser in a vacuum. However, this necessitates a complicated and expensive encapsulation. The mirror faces may also be covered with a transparent dielectric protective layer. However, it is technologically difficult to provide a readily adhering protecting layer which is impervious to water vapor and has the correct thickness. A readily adhering and impervious protecting layer can be obtained inter alia by thermal oxidation of the mirror faces. However, this presents great problems in connection with the required high temperatures. Since the oxidation would have to be carried out after the metallization of the laser so that the oxide does not have to be removed for providing electrode layers, the oxidation temperature is restricted to that which the metallization can stand. Undesired impurities from the metallization may also enter the laser and the oxide. Furthermore, undesired diffusion within the laser structure may take place. The use of oxidized mirror faces is therefore to be discouraged.
From the publication JP-A53-61985 a method is known in which an improved thermal oxidation of the mirror faces is achieved by bombarding the mirror faces, prior to the oxidation, with ions, for example protons, so as to form an implanted layer having a thickness smaller than the internal laser wavelength. The above-mentioned disadvantages associated with thermal oxidation of the mirror faces are not obviated by this method.
It is also possible to provide dielectric protecting layers by sputtering or vapor deposition. However, in this manner also an efficacious protection of the mirror faces is technologically not easy to realize.
From the publication JP-A55-18078 a method is known in which on etched mesa mirror faces a dielectric layer is grown which is then bombarded with protons so as to improve the resistance against erosion. However, this method requires at least an extra etching step and the growth of a dielectric protective layer.
Another solution can be found by providing the active region with end zones which adjoin the mirror faces and have larger forbidden band gaps in which substantially no radiation absorption and consequently little non-radiating recombination occurs, as described in the above-mentioned article by Yonezu et al. in Applied Physics Letters 34 (1979) pp. 637-639. The selective zinc diffusion mentioned in this article, however, is complicated and not fully effective.